1. Field of the Invention
The present invention relates to .theta.-alumina powder, a method for preparing the .theta.-alumina powder, and a magnetic recording medium comprising the .theta.-alumina powder as an abrasive.
2. Description of Related Art
Generally, a magnetic recording medium for an audio, video and computer has a layer made of a magnetic paint which is prepared by dispersing a magnetic material such as ferromagnetic metal oxides or metal powder in a binder and coated on a non-magnetic substrate. Since the magnetic recording medium is used with contacting while being contacted to a magnetic head, the magnetic layer should have a sufficient travel property and durability.
When the durability is insufficient, particles dropping from the magnetic layer adhere around the magnetic head to prevent a smooth contact of the magnetic layer with the magnetic head. Accordingly, an electromagnetic conversion property is deteriorated, a drop out arises and a tape is significantly contaminated during still reproduction. When the travel is insufficient, friction of the tape varies to deteriorate the travel property.
Hitherto, inorganic powder such as alumina, silicon carbide, chromium oxide, titanium oxide, silicon oxide and .alpha.-iron oxide has been added as an abrasive to the magnetic layer so as to improve the travel property and durability of the magnetic layer.
Alumina which is often used as the abrasive is predominantly .alpha.-alumina. It is known that aluminum hydroxide such as gibbsite, bayerite and boehmite can be converted to intermediate alumina and then .alpha.-alumina by heating as follows: .chi..fwdarw..kappa..fwdarw..alpha., .gamma..fwdarw..delta..fwdarw..theta. .fwdarw..alpha., .eta..fwdarw..theta..fwdarw..alpha., .rho..fwdarw..eta..fwdarw..theta..fwdarw..alpha., pseudo .gamma..fwdarw..theta..fwdarw..alpha. alumina (cf. for example, "Electrochemistry" (Japan), Vol. 28, page 302 by Funaki and Simizu; On alumina hydrate and alumina, Table 1, Examples of thermal change of alumina hydrate ).
It is also known that amorphous alumina can be converted through intermediate alumina such as .gamma., .beta. and .theta.-alumina to .alpha.-alumina by pyrolysis of an aluminum salt such as aluminum chloride, aluminum sulfate and aluminum nitrate (cf. for example, "Mineralogy Journal" ( Japan ), Vol. 19, No. 1, page 21 and 41).
The conversion from intermediate alumina to .alpha.-alumina is an exothermic reaction, and the grain growth of .alpha.-alumina is faster than that of intermediate alumina. .alpha.-Alumina used as the abrasive for the magnetic recording medium has a high Mohs hardness of 9 to contribute the improved travel property and durability of the magnetic layer. However, it is impossible to prevent incorporation of the coarse particles prepared by the grain growth curing the phase transition into .alpha.-phase, so that the coarse particles deteriorate a tape surface smoothness, scratch the magnetic head during the travel, and prevent a smooth contact with the magnetic head to deteriorate the electromagnetic conversion property.